1. Field of the Invention
This invention relates to the preparation of projection-viewable transparencies by an electrographic copy process. In one aspect, this invention relates to an electrographic copy process in which an image pattern of fusible toner particles is fused onto an image-receiving hydrophilic colloid layer of a transparent receiver element by contacting the element with a heated fusing surface coated with a release liquid that prevents undesirable transfer of toner particles to the fuser surface. In another aspect, this invention relates to projection-viewable transparencies that are formed in such process.
2. Description of the Prior Art
It has been known for many years that the projection of an image present upon a transparency may serve as an effective means for conveying information to one or more observers. Such transparencies can be formed by a number of methods, a common one being transfer electrostatic copying. By this process, an image of fusible toner particles is formed on a receiving layer of a transparent element. The particles are then fixed to the element by contact with a heated fusing surface such as a roller which is coated with a release liquid to inhibit transfer or "offsetting" of toner particles from the element onto the fusing surface.
Prior art transparencies are composed of a transparent film support and an insulating receiving layer on one or both sides of the support for receiving the toner particles. Typical receiving layers are hydrophobic layers formed from a wide variety of materials including polyamides (U.S. Pat. No. 3,535,112, issued Oct. 20, 1970 to T. J. Dolce et al.); vinylidene chloride copolymers (U.S. Pat. No. 3,539,340, issued Nov. 10, 1970 to T. J. Dolce et al.); poly(vinyl butyral); poly(bisphenol A carbonate); polystyrene; polyesters of terephthalic acid, ethylene glycol and 2,2-bis[4-(.beta.-hydroxyethoxy)phenyl]propane; poly(vinyl formal); vinyl chloride-acrylonitrile copolymers; vinyl chloride-vinyl acetate copolymers; poly{4,4'-(2-norbornylidene)diphenylene carbonate} (British Pat. No. 1,237,386, published June 30, 1971 in the name of Eastman Kodak Company); poly(ethyl methacrylate); mixed acrylic polymers containing methyl and butyl methacrylate, butyl acrylate and a small amount of either a carboxylate salt or melamine-formaldehyde material. Prior art transparencies that are prepared according to the aforementioned copy process may also have receiving layers that are provided with surfactants, wetting agents and the like which are capable of rendering the receiving surface hydrophilic. Typical examples of such transparencies are tinted Arkwright PPC Transparency Films (Arkwright, Inc., Fiskeville, R.I., 02823) and those disclosed in U.S. Pat. No. 3,549,360 (issued Dec. 22, 1970 to A. J. O'Neill et al.).
As will be apparent from the discussion hereinafter, a transparency formed by an electrographic process should have certain characteristics to render it particularly useful in conveying information to one or more observers. For example, substantially clear non-toned areas, resistance to abrasion in toned areas and ability to selectively remove information by simply rubbing with a damp cloth or tissue are characterisitics of considerable importance. The importance of substantially clear non-toned areas in a transparency is apparent. Resistance to abrasion in toned areas is needed so that a transparency can withstand conventional handling conditions without damage to and loss of information in toned areas. The ability to selectively remove information from a transparency is important in order to illustrate particular points of interest to a viewing audience and to provide flexibility in using such a transparency. In this regard, removing such information by simply rubbing with a damp cloth or tissue, as described herein, is convenient and avoids possible damage to a transparency which can occur when such information is removed by scraping. It is also desirable to prepare a transparency having the aforementioned combination of characteristics using an electrographic process that can be operated over a wide range of processing conditions. Thus, it is important to be able to prepare such a transparency without being unduly limited to the specific toner fusion temperatures of a particular commercial electrographic copier.
Unfortunately, the state of the prior art has not advanced sufficiently to the point where a transparency having the aforementioned combination of properties can be prepared in an electrographic process using a wide range of conditions. Thus, we have observed that prior art transparencies having a surfactant coated on the image-receiving layer perform quite differently at different toner fusion temperatures. For example, at temperatures of about 340.degree. F. (171.degree. C.) such transparencies exhibit low resistance to abrasion in toned areas. At temperatures of about 375.degree. F. (191.degree. C.) the same transparency is more resistant to abrasion in toned areas, but exhibits undesirable haze in non-toned areas. Furthermore, regardless of the toner fusion temperature employed, toned areas of such transparencies cannot be removed by light rubbing with a wet cloth or tissue. Moreover, we have also observed with prior art transparencies having receiving layers composed of hydrophobic materials such as polyethylene terephthalate, that release liquid employed during fusion accumulates in irregular patterns on the receiving layer. This release liquid appears as an unsightly stain in non-toned areas when the transparency is projection viewed. In addition, the toned areas in such transparencies cannot be removed with a wet cloth or tissue.